The present invention relates to an ionizing radiation curable resin composition suitable for the production of optics such as Fresnel lenses. The present invention also relates to an ionizing radiation curable resin composition having excellent adhesion to substrates and a transmission screen having a Fresnel lens sheet provided with a Fresnel lens comprising a cured product of the ionizing radiation curable resin composition.
Conventional transmission screens for projection televisions have, for example, a construction comprising a Fresnel lens sheet for transforming diffused light from light sources, such as CRT, to parallel light and transmitting the parallel light to a viewer, in combination with a lenticular lens sheet for distributing light only to the viewing range of the viewer in order to effectively utilize a given quantity of light.
In recent years, projection televisions have been more and more large-sized. In these projection televisions, in order to reduce the thickness of televisions per se, a lens sheet, with a short focal point, comprising a material having a high refractive index has been used as a Fresnel lens sheet to shorten the distance between the light source and the screen and to project light emitted from the light source onto the screen at a large angle and so as to minimize the reflection loss.
For the formation of lenses, with a high refractive index, having a short focal point for transmission screens, a resin sheet of a methyl methacrylate/styrene copolymer (refractive index n=1.56 to 1.57) prepared by copolymerizing highly transparent polymethyl methacrylate (refractive index n=1.49) with polystyrene having a high refractive index (refractive index n=1.59) or the like has been provided followed by molding of the surface of the sheet by heat pressing or the like into a Fresnel lens to prepare a Fresnel lens sheet.
The formation of large-area Fresnel lens sheets by the conventional heat pressing suffers from problems including that close temperature control in heating is necessary for reproduction of complicated surface shapes, molding is troublesome, and pressing and cooling require a lot of time leading to remarkably lowered productivity.
For this reason, a molding method alternative to the conventional heat pressing technique has been proposed wherein an ultraviolet curable resin is used to form a Fresnel lens on a substrate so that a Fresnel lens sheet can be efficiently formed in a short time simply by irradiation of the Fresnel lens with an ionizing radiation, such as ultraviolet light (Japanese Patent Laid-Open Publication No. 9301/1991).
In the case of conventional ionizing radiation curable resin compositions for Fresnel lenses, increasing the refractive index of cured products thereof increases the hardness of the cured products and results in lowered abrasion resistance. Therefore, in this case, when the lenticular lens sheet is used in combination with the Fresnel lens sheet, the surface of the Fresnel lens sheet is unfavorably scratched due to vibration and the like in transit to create scratch marks. On the other hand, adoption of a flexible composition from the viewpoint of enhancing the abrasion resistance of the surface of the Fresnel lens sheet results in a lowered refractive index. Thus, the conventional ionizing radiation curable compositions cannot simultaneously satisfy surface abrasion resistance and refractive index requirements.
Japanese Patent Laid-Open Publication No. 117348/1993 proposes a composition for solving the above problems. This composition, however, contains bromoacrylate from the viewpoint of improving the refractive index. This bromoacrylate is a halogen compound which, when burned as a waste, raises a problem of environmental protection. Further, this composition has low adhesion to the substrate of Fresnel lens sheets. Therefore, a primer layer should be provided in order to improve the adhesion to the substrate. This complicates the production process.
Accordingly, it is an object of the present invention to provide an ionizing radiation curable resin composition for a Fresnel lens that has high refractive index, possesses excellent abrasion resistance, and is used for the formation of Fresnel lens sheets having excellent adhesion to synthetic resin substrates. It is another object of the present invention to provide a transmission screen using the composition.
According to one aspect of the present invention, there is provided an ionizing radiation curable resin composition for a Fresnel lens, comprising the following components (A) to (D), said radiation curable resin composition providing, upon curing, a cured product having a refractive index of not less than 1.55:
(A) a bisphenol A epoxy(meth)acrylate represented by chemical formula 1 
xe2x80x83wherein R represents H or CH3 and n is an integer of 1 or more;
(B) a monofunctional (meth)acrylate;
(C) a bifunctional (meth)acrylate; and
(D) a polymer.
Preferably, the composition of the present invention comprises 10 to 50 parts by weight of the bisphenol A epoxy (meth)acrylate, 10 to 50 parts by weight of the monofunctional (meth)acrylate, 10 to 50 parts by weight of the bifunctional (meth)acrylate, and 1 to 20 parts by weight of the polymer.
According to another aspect of the present invention, there is provided a transmission screen having a Fresnel lens sheet, the Fresnel lens sheet comprising a substrate, of a transparent resin composed mainly of methyl methacrylate, a polystyrene resin, or a polycarbonate resin, having thereon a Fresnel lens comprising a cured resin obtained by curing of the above composition.